Membranes selective for metal ions are used for example in ion-selective sensors which are used for measuring the concentration or activity of a certain ion type in an electrolyte solution. The operation is based on the occurrence of a potential difference across the membrane selective for metal ions that separates the test solution from a reference solution. Examples of such sensors are the ION Selective Field Effect Transistor (ISFET), electrode, optrode, coated-wire electrode, and sensors based on planar silicon technology.
For the present invention the ISFET is of particular importance. A number of membranes are arranged on the gate region of an ISFET. In addition to a hydrophilic membrane, the so-called hydrogel, there is a hydrophobic membrane. The electro-active components are included in the matrix of this latter membrane. On the one hand these are the ionic sites which determine the so-called perm-selectivity, that is, the selectivity for either anions or cations. On the other hand there is a selectivity-determining group which determines the selectivity of the membrane for a specific ion.
The selectivity-determining group included in a membrane matrix can for instance be an ion exchanger or a neutral ionophore. To indicate bivalent cations, ionophores which are based on diamides, in particular dioxaoctane diamides, are usually employed. In the development of such dioxaoctane diamides two properties are important, namely on the one hand the selectivity which determines which (bivalent) ion is bonded, and on the other the lipophilicity which determines the speed at which the ionophore is washed out of the membrane and thereby determines the useful life of the sensor.
The selectivity can be influenced using substituents which determine the relative position of the coordinating ligands O, S, and SO, or through the choice of the type of coordinating ligand: O, S, or SO.
With known dioxaoctane diamide derivatives the lipophilicity is often influenced by arranging long alkyl chains on the amide groups. Such long alkyl chains form as it were an anchoring in the membrane matrix which prevents washing out of the ionophore. It is assumed by some that arranging such long alkyl chains also influences the position of the coordinating ligands and thereby the selectivity.